Otoacoustic emissions (OAEs) are acoustic responses that originate from the inner ear and can be measured with an insert microphone placed in the ear canal. These responses can be recorded quickly and noninvasively, and cannot be measured from ears having as little as 25 dB of hearing loss. For these reasons, OAEs are being advocated by many researchers as a possible test for diagnosing hearing loss in infants. Many centers are currently investigating the possibility of using OAEs in neonatal nurseries to screen for hearing loss, but little attention has been directed to understanding maturational changes of OAEs in infants to the age of 2 years. It is essential that a better understanding of OAEs in infants be gained to be maximally beneficial in this population. The main purpose of the proposed research is to study OAEs longitudinally from birth to two years of age in humans. The rationale for these studies is that preliminary data as well as those from other researchers indicate that transient-evoked, distortion product and spontaneous OAEs measured in infants have greater levels and higher frequency content than those measured in older children, even those aged only 2 years, and adults. The hypothesis motivating these studies is to investigate the possibility that the cochlea itself continues to develop post-natally. OAEs are thought to be a reflection of biomechanical activity from the cochlea, and thus, are an excellent measure to test whether the cochlea, independent of maturation of mylenization, matures post-natally. Research has also shown that multifrequency tympanograms change with age, which has been an indication of developmental changes of the middle ear. Correlations between changes in multifrequency tympanograms and changes in OAEs in individual subjects will suggest which changes in the OAEs could be due to influences of middle ear maturation. Correlations between DPOAEs and COAEs will also be tested which will lead to better understanding of evoked OAEs. One practical side issue will result from these experiments, which is to determine optimal stimulus parameters to evoke DPOAEs in infants, a clinically useful outcome. In combination, these studies will provide information about development of the auditory system in infants and measures that will lead to identification of hearing loss at a very critical period in human life.